Ellis stood unsuccessfully as the Conservative candidate in the Park ward of Northampton Borough Council in 1995.[6] However, he was elected in 1997 as a Conservative Councillor on Northamptonshire County Council, representing the Northampton Park (now Parklands) Ward, winning the seat from his Labour Party rival by just 44 votes. He served until the next election in May 2001, when he did not stand again.[7] At the time of his election he was the youngest County Councillor in Northamptonshire, at the age of 29.[8]

In November 2010, Ellis established an All Party Group on the Queen's Diamond Jubilee, which he chaired for the following three years.[14] In this role, Ellis was responsible for organising a gift of a stained glass window of The Queen's Coat of Arms for the Queen from both Houses of Parliament.[15] Ellis was also responsible for organising the planting of a Red Windsor apple tree on Speaker's Green at the Houses of Parliament as part of the Woodland Trust's Jubilee Woods project.[16]

In July 2010, he was first elected onto the Statutory Instruments (Joint Committee) and worked on the Draft Communications Data Bill during the 2012–13 Parliamentary session.[17] Ellis was interviewed about this Bill with Jimmy Wales, co-founder of Wikipedia, on the Daily Politics programme on 11 December 2012.[18] In February 2011, Ellis was first elected onto the House of Commons Home Affairs Select Committee.[19][20] Writing in The Independent newspaper, Ian Burrell described Ellis as asking questions in one case with "all the gravitas of a prosecuting counsel".[21]

On 11 September 2013, Ellis introduced the Medical Innovation (No.2) Bill, a Private members bill to the House of Commons.[23][24] The bill was designed to allow doctors more scope to innovate when treating cancer patients, but was heavily criticised by a range of medical and legal bodies, patient groups and charities.[25][26] The bill was withdrawn after its first reading,[27] following an indication from the government that they would support it.[28] Although the Conservative MP Dan Poulter MP, who was Parliamentary Under Secretary of State at the Department of Health, suggested in July 2014 that the Government was keen to support it, it failed to progress through the House of Commons after the Liberal Democrats declined to support it.[29]

In March 2014, the Chancellor of the Exchequer, George Osborne announced in the House of Commons during the Budget Speech that a campaign Ellis had been conducting to secure extra funds to reduce potholes had succeeded and that a £200 million fund was being created to be distributed nationwide.[30] In June 2014, it was announced that £3.3 million of this fund would be allocated to Northamptonshire by the Department for Transport, with various repairs in Northampton.[31][32]

In July 2014 Ellis was successful in calling for the Parliamentary authorities to officially mark the assassination of a former Member of Parliament for Northampton, Spencer Perceval, who had become Prime Minister, and who was shot and killed in the House of Commons in 1812. The Parliamentary authorities agreed to install a brass plaque in St Stephen's Hall commemorating the notable assassination and Ellis called this a "fitting tribute" to the former Prime Minister and historic Northampton figure.[33]

In February 2018, following the announcement that Northamptonshire County Council had brought in a "section 114" notice, putting it in special measures following a crises in its finances, Ellis was one of seven local MPs who released a statement arguing that the problems with the authority were down to mismanagement from the Conservative councillors who led it rather than funding cuts from the Conservative Government. They further argued that government commissioners should take over the running of the Council.[38]

In the House of Commons he has sat on the Statutory Instruments (Select and Joint Committees) and the Home Affairs Committee.[36] He is a member of the Conservative Friends of Israel group, and has participated in delegations to raise concerns about an agreement relating to Iran's nuclear capabilities.[39]

In May 2016, it emerged that Ellis was one of a number of Conservative MPs being investigated by police in the United Kingdom general election, 2015 party spending investigation, for allegedly spending more than the legal limit on constituency election campaign expenses.[40] However, in May 2017, the Crown Prosecution Service said that while there was evidence of inaccurate spending returns, it did not "meet the test" for further action.[41]

In July 2017, Ellis was called as a witness in the trial of Adam Simmonds, the former Conservative Police and Crime Commissioner for Northamptonshire, who faced charges of leaking information relating to fraud allegations against Peter Bone, the Conservative MP for Wellingborough. Ellis admitted a 'hazy recollection' of 'discussing a criminal investigation into Peter Bone and the damage it might do to the Conservative Party with Adam Simmonds over coffee'. Neither Simmonds or Bone were ultimately found guilty of the separate charges brought against them.[42]

In March 2018, Ellis was criticised by local campaigners over the cuts to library services in Northampton, given his role as the Government minister for libraries. Criticism followed the announcement that 21 book-lending services were at risk of closure in Northamptonshire, after the Conservative run County Council cut £40 million from its budget. Ellis responded that the 'Northamptonshire Libraries Friends Groups and Supporters' were "attempting to bring party politics into this issue and that he had been a long-standing critic of the leadership of the Council.[43]

^[saatchi-bill-medical-innovation _title=Attacking critics is no way to fix the Saatchi bill "Attacking critics is no way to fix the Saatchi bill"] Check |url= value (help). The Guardian. 19 November 2014. Retrieved 15 August 2018.